Post by Steaphany on Jul 3, 2013 23:20:05 GMT
From "Photographer's Mate 2 & 3", Bureau of Naval Personnel, Rate Training Manual, NAVPERS 10355-A, 1971 Edition, "The DARKROOM COOKBOOK" Third Edition by Steve Anchell, Kodak's "Processing Chemicals & Formulas for Black-and-White Photography", J-1 Professional Data Book, 1977, and other sources:
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Developing Agents | Preservatives | Accelerators | Restrainers |
| The developing agent ( reducing agent ) is the most important chemical in a developing solution. Nevertheless, the other ingredients are necessary to make the solution function properly. Although several chemicals are capable of developing Silver Halides to metallic Silver, relatively few of them can be used. Many chemicals tend to develop the unexposed as well as the exposed Silver Halides; that is, some are not selective in their action.The few developing agents that are used vary greatly in their properties. One of the properties of a developing agent is it's reducing potential. This refers to it's relative ability to develop or reduce the Silver Halides.A developing agent of high potential attacks Silver Halides vigorously, whereas, one of low potential is slower to take action. The characteristic activity of a developing agent is another factor to consider. Some agents are more active in the strongly exposed areas, and others have an equal overall activity. The temperature of the solution affects the activity of some agents much more than others. The tone of the developed image is greatly affected by the type of developing agent. Some agents produce blue-black or cold tones, while others yield brownish or warm tones. Hydroquinone, Metol, Amidol, glycin, paraphenylene diamine, and pyro are some of the more widely known developing agents. HYDROQUINONE - A developing agent of low potential. It requires little to no restrainer at normal temperatures. It's activity is greatly reduced by low temperature because it becomes inactive ( inert ) at about 10ºC in normal solutions. At temperatures of about 27ºC, it works so energetically that it develops th unexposed parts of an emulsion. This produces fogging of the image which is the term used to describe an image that has developed Silver in it's unexposed portions. No other developing agent is as sensitive to temperature as Hydroquinone. When Hydroquinone is used in solutions at normal working temperatures ( 20ºC ), it is capable of producing highlights of great density while retaining transparency in the unexposed areas. It preferentially works first on the areas of greatest exposure, the highlights, and does not begin on the lesser exposed areas until they equal what remains to be developed in the heavily exposed areas. The areas of least exposure, the shadows, are last to be developed. This makes it an ideal developing agent for subjects requiring requiring extreme contrast - reproductions of black lines on a white background. Therefore, Hydroquinine is said to build contrast first and density afterward. Hydroquinone deteriorates slowly in air, has good keeping qualities on solution, and does not stain gelatin. For subjects that require accurate rendering of highlights, mid tones, and shadows, it is combined with another developing agent such as Agfa Metol ( P-Methylaminophenol Sulfate ). See MQ Developers below. Metol, Elon, Pictol, Rhodol - These are trade names for another very good developing agent - P-Methylaminophenol Sulfate. Metolalone or in combination with Hydroquinone has been one of the most popular developing agents since it's introduction, which was prior to 1900. It works at the same speed in all parts of the image. It completes the development of the shadows, or areas of least exposure, soon after development starts. The areas of greatest exposure are the last to be completely developed. Metol is a high potential, soft working developing agent which is affected comparatively little by changes in temperature or by large amounts of a restrainer in the solution. It's characteristic action is to build density first and then produce contrast. Although Metol develops with out an accelerator, it is generally used with an alkaline carbonate; sometimes Borax, Sodium Borate, and other alkalis are utilized for special purposes. Under some conditions it is difficult to obtain sufficient contrast with Metol alone, hence it is usually combined with Hydroquinone. Other combinations are Metol-glycin and Metol-pyro. Metol is used in fine grain developers, in developers ( with Hydroquinone ) for prints and negatives, and in solutions with pyro for negatives. Metol solutions have good keeping qualities. Metol-Hydroquinone, abbreviated MQ, is the most versatile and popular of all developers. The soft working, detail producing Metol and the high contrast Hydroquinone make a combination superior in many ways to either agent when used alone. MQ Developer keeps well in solution, does not stain, and is faster working than either Metol or Hydroquinone used separately. A combinaton of Metol and Hydroquinone can be selected for practically all photographic developing solutions. When used at nermal temperatures, the results are almost proportional to the amount of exposure, Many packages developers are made up from these two agents. DIAMINE - p-Phenylenediamine has a low potential. It produces negatives of good tonal range, fine grain, and warm tones. Slight increases in temperature have little effect on it's activity. It is somewhat toxic and may cause skin irritation. | All organic developing agents in an alkaline state have a strong affinity to Oxygen. It is necessary, therefore, to add a preservative to developing solutions to prevent excessive oxidation. The preservative prolongs the useful life of the developing solutions and prevents the formation of colored oxidation products which cause stains. The preservative is a chemical which has a very great affinity for free Oxygen, and combines with it when mixed into a solution. A large amount of free Oxygen os in the air and the water used for solutions. If the free Oxygen is left in the water, it Oxidizes most of the developing agent and produces stains before the metallic silver image is satisfactorily developed. By adding a significant quantity of preservative, practically all the free Oxygen is removed from the solution, the developing agent works as intended, and no stains are produced. Sodium Sulfite is the preservative most commonly used. It also dissolves Silver Halides to some extent, reduces grain size, and therefore useful in fine grain developers. Sodium Bisulfite is also used. It is an acidified Sulfite which, in an alkaline developer, is converted to Sodium Sulfite and Sodium Bicarbonate. Developers with Sodium Bisulfite give slightly less base fog than those containing Sodium Sulfite. The quantity of preservative used varies greatly as shown by the following factors which determine the amount required: 1. The tendency of the developing agent to oxidize. 2. The concentration of the developer. A dilute developer requires more preservative than a more concentrated developer. 3. The temperature at which the developer is stored or used. The rate of oxidation increases as the temperature increases. 4. The keeping properties required and the way the solution is used. A solution that is to be used only once and then discarded requires very little preservative. Furthermore, oxidation is more rapid when a developing solution is used in a tray than when it is used in a tank. 5. The alkalinity of the solution. The more strongly alkaline the developer, the more rapid is the rate of oxidation. | All developing agents are either neutral or slightly acid and, as such, they usually have little developing ability. In order to utilize the developing capabilities of these agents, it is necessary that they be in an alkaline state. To make the developing solution alkaline, an accelerator is added. More to come... | Without a restrainer most developing solutions act too rapidly and develop unexposed Silver Halides near the surface of the emulsion . This causes chemical fog, developing streaks, and an image lacking contrast. When a restrainer is added, development time is prolonged and fog is minimized. Contrast increases because the developing agent's activity is cut down in unexposed areas. However, excessive amounts of restrainer greatly retard development.The chemical most commonly used as a restrainer is Potassium Bromide. More to come... |
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